There exist numerous Human Machine Interface (HMI) devices designed to sense the presence of human touch today. In some cases these HMI interfaces include a stylus that is used to provide input from the human to the machine interface. A stylus may completely replace the direct human interface or may supplement the human interface. These HMI devices may use light, sound, mechanical-electro (switches) magnetic fields, electric fields, electromagnetic fields, or a combination of these stimuli.
Three prior and current touch technologies that exist today and that use electric fields are commonly referred to as projected capacitance, capacitive, and differential sensing. Projected capacitance is commonly associated with transparent touch screens that are used in conjunction with displays of the same approximate size and are assembled with such displays in a manner as to allow the light from the display to pass through the sensing elements of the projected capacitance touch screen sensing elements. Projected capacitance is usually implemented with high resolution capabilities where the selection of an area of touch can be much smaller than the actual size of a finger. Projected capacitance is widely used on personal electronic devices such as cell phones, personal digital assistants (PDAs), smart phones, notebooks, laptop computers, laptop monitors, and other user devices that have displays. Capacitance sensing, as opposed to projected capacitance, is usually applied in applications where singular inputs are processed that generally respond to much lower resolution than projected capacitance, such as buttons or low resolution sliders. These lower resolution input sensing applications use electrode structures that are designed to respond to a finger sized input. Nonetheless, capacitance sensing can be used in place of projected capacitance, and in principle projected capacitance is a subset implementation of capacitance in general. Differential sensing technology uses electric fields, low impedance sensing techniques, and unique sensing electrodes that in conjunction with specific electronic sensing circuits allow for the accurate, robust sensing of human touch without the use of software.
Capacitance, projected capacitance, and differential sensing have at least two common attributes: 1) they all use electric fields as the stimulus for measuring the human machine interaction and 2) they rely on a predetermined threshold that is determined by the engineer which corresponds to a touch when a certain stimulus change has occurred due to human machine interaction.
FIGS. 1 and 2 illustrate basic single input sensor configurations for using multiple electrode and single electrode capacitance sensing. FIG. 2 illustrates a simple capacitance sensor with a single electrode 100 for sensing through a dielectric substrate 102. The touch stimuli would be inserted on the opposite side of the dielectric 102 of which the single electrode 100 is located. FIG. 1 illustrates a multiple electrode capacitive sensor having a dielectric substrate 102 and at least two electrodes 100, 104. Similarly to FIG. 2, the touch stimulus would be inserted on the opposite side of the dielectric 102 of which the multiple electrodes 100, 104 are located. These capacitance sensing techniques related to the structures in FIGS. 1 and 2 above sense changes in capacitance from single or multiple electrodes in such a manner that after the stimuli signal is processed there will be an output signal that will change as a finger or stylus approaches the sensing electrode(s). The output signal is processed in such a way that when a certain value is reached (predetermined threshold) a touch response will occur. This predetermined threshold would correspond to a touch position located with a touch zone above the touch surface. Changes—affected by manufacturing tolerances, the dielectric constant, the dielectric thickness, the electrode area, and the electronic sensing circuit variances—will cause the actual touch location above the sensor electrode(s) to also vary.
Refer to FIGS. 3 through 7. FIG. 3 illustrates an electrical schematic and block diagram of a single electrode capacitance sensor as illustrated in FIG. 2 and timing diagrams illustrated in FIGS. 4 through 7, a basic technique for detecting and processing a touch input utilizing a single electrode. Ce represents the effective net capacitance of a single electrode sensing element, illustrated in FIG. 2. Ce will change depending on the capacitance present, i.e. with “no touch” Ce will have lower value of capacitance than when a “touch” is present in which case Ce will have a higher value of capacitance. Cs represents a sampling capacitor for the Analog to Digital Converter 106, Pre-Determined Threshold Circuitry 108, and Output Response 110. Control devices A, B, and C represent electronic switches where when they are turned on will be in minimal resistance mode (ideally, zero ohms) and when off are in high resistance mode (ideally, infinite resistance).
FIGS. 4, 5, 6, and 7 are timing diagrams used to describe the basic operation of a sensing technique for sensing a touch input using a single electrode Ce. FIG. 4 illustrates the timing diagram for a control signal for control device A. When the control signal is at a value of 3.00 the control device is on and when the control signal is at a value 0.00 the control device is off. The same hold trues also for control signals for control devices B and C in FIGS. 5 and 6. At time t1 in FIG. 6 control signal C goes high causing control device to turn on connecting Ce to Cs. Also at time t1 control signals A and B are low as indicated turning off control devices A and B. At time t2 control device A is turned on discharging any charge that is present on Ce and Cs ground as indicated by the voltages Vs dropping to 0.00 from a voltage value of 1.00 in FIG. 7. At time t3 control device A is turned off. At time t4 control device C is turned off isolating Ce from Cs. At time t5 control device B is turned on charging sampling capacitor Cs to Vdd. FIG. 7 illustrates the voltage Vs charging from a value of 0.00 to a Vdd value of 3.00. At time t7 control device C is turned on connecting Cs to Ce causing the charge on Cs to redistribute to both Cs and Ce and therefore the voltage Vs to drop proportional to the amount of capacitance on Ce. The capacitance of Cs is constant. The lower voltage will drop according to the below equation:Vs=Vdd*(Cs/(Cs+Ce))At time t7 the “no touch” value of 1.00 is illustrated in FIG. 7. If there were a touch event, the capacitance Ce would be at higher value than the “no touch” capacitance value. Based on the above stated equation, Vs is shown as a lower value of 0.500 in FIG. 7. At time t8 the control device C is turned off disconnecting the sensor capacitor Ce from the sample capacitor Cs. The value of Vs would remain at the sampled value that is proportional to the touch condition, a higher value for “no touch” condition and a lower value for the “touch” condition.
An alternative capacitance detecting technique utilizing multiple electrodes is described here. Refer to FIGS. 1, 8 through 12. FIG. 8 illustrates an electrical schematic and block diagram of a multiple electrode capacitance sensor as illustrated in FIG. 1, and timing diagrams illustrated in FIGS. 9 through 12, a basic technique for detecting and processing a touch input utilizing a multiple electrodes. Ce represents an effective net capacitance for a multiple (two) electrode sensing element, illustrated in FIG. 1. Ce will change depending on the capacitance present, i.e. with “no touch” Ce will have higher value of capacitance and when “touch” is present Ce will have a lower value of capacitance. Cs represents a sampling capacitor for the Analog to Digital Converter. Control devices A and C represent electronic switches where when they are turned on will be in minimal resistance mode (ideally, zero ohms) and when off are in high resistance mode (ideally, infinite resistance). Control device B is represented as a MOSFet circuit for generating a drive signal on the output of control device B. FIGS. 9, 10, 11, and 12 are timing diagrams used to describe the basic operation of a sensing technique for sensing a touch input using a multiple electrode capacitance sensor Ce. FIG. 11 illustrates the timing diagram for a control signal for control device C. When the control signal is at a value of 3.00 the control device is on and when the control signal is at a value 0.00 the control device is off. The same hold trues also for the control signal for control device A in FIG. 9. FIG. 10 illustrates the timing diagram for the output drive signal B which varies from a value of 0.00 to a value of 3.00.
At time t1 in FIG. 11 control signal C goes high causing control device C to turn on connecting Ce to Cs. Also at time t1 control signal A is low turning off control device A and output B is low, both states shown in FIGS. 11 and 10 respectively. At time t2 control device A is turned on discharging any charge that might be stored on Ce and Cs to ground as indicated by the voltage Vs dropping to 0.00 from a voltage value of 1.00 in FIG. 12. At time t3 control device A is turned off. At time t4 output device B is turned on causing the voltage applied to sensor electrode structure from a value of 0.00 to 3.00. The voltage stimulus will cause the value of Vs to rise to a value that is proportional to the capacitance of Ce as shown by the voltage rising from 0.00 at to a value of 1.00 for a “no touch” condition. If there were a finger/appendage or other touch input device to approach or come into contact with the touch surface, then the capacitance of Ce would be at a lower effective capacitance for a “touch condition” causing the voltage to be at Vs to settle at a lower value as indicated by the value of 0.500 at the “touch condition”. Both of these conditions are illustrated in FIG. 12. At time t5 control device C is turned off isolating Ce from Cs. At time t6 output device B goes low removing stimulus from the electrode structure Ce. The capacitance of Cs is constant. The lower voltage will drop according to the below equation:Vs=Vdd*(Ce/(Cs+Ce)).At time t6 the “no touch” value of 1.00 is illustrated in FIG. 12. If there were a touch event, the capacitance Ce would be at higher value than the “no touch” capacitance value. Based on the above stated equation, Vs is shown as a lower value of 0.500 in FIG. 12 capacitor Ce from the sample capacitor Cs. The value of Vs would remain at the Vs value that is proportional to the touch condition, a higher value for “no touch” condition and a lower value for the “touch” condition. One useful attribute of this dual electrode sensing technique is that if water were to lie on the touch surface of the touch sensor structure, Ce would essentially go up in value, then causing Vs to increase in value. This is useful in that the Vs moves in the opposite direction for water as compared to a normal touch event. This information is very useful in inherently discriminating against false touch events do to water laying on the touch surface.
In both cases above, whether single electrode or dual electrodes, the analog to digital converter 106 would convert the value of value of Vs to a digital value that can be processed by the Predetermined Threshold Processing Circuit 108. Two examples of how a Predetermined Threshold Value would be determined might be: 1) the Predetermined Threshold Value equals a Voltage value where when Vs is equal to or less than that that same said Voltage value then there is a valid touch event, i.e. valid touch event is present when V (sample)<=Vp (predetermined threshold value), or 2) the Predetermined Threshold Value equals a Voltage value where when difference between the “no-touch” Vs value and the Vs is equal to or greater than that same said Voltage value then there is a valid touch event, i.e. a valid touch event is present when [(the value of a the “no touch” voltage)−(Vs)]>=V (predetermined threshold value). Threshold Processing Circuitry 108 will take the digital representation of the Vs and the Threshold Processing Circuitry 108 will then, using Predetermined Threshold Value processes similar to that described above, process and decide if there is a valid touch event to be processed by the Output Response circuit 110 for proper interfacing to the outside world. The value for the Predetermined Threshold Value must be determined by the designer of the application of capacitance or field effect sensor. The Predetermined Threshold Value is a value that ultimately is compared to a sampled value that is proportional to the touch stimulus that is then interpreted as a touch event. There are numerous techniques that have been developed that would use this method of using a Predetermined Threshold Value. Even differential sensing techniques using multiple sensing electrodes require that the value sensed on one set of electrodes have some value relative to other sets of electrodes, e.g. as an example in a differential two electrode sensing structure both electrodes may need to be equal to each other in order for there to be a touch event and one of the electrodes may need to be less than the other for there to not be a touch event (logically NOT touch). Regardless of the technique, when using Predetermined Threshold techniques, there are other variables that can ultimately affect the value of sampled voltages such as Vs in FIGS. 7 and 12, other than the “no touch” or “touch” events. Changes in the dielectric constant of the touch substrate, effective variances in sensor pad area, variances in area of finger coupling to the sensor structure, variances based on tolerances of glass substrate, the variance in the sampling circuitry, temperature, moisture, etc. can all lead to false or under/over sensitive touch sensing response. FIGS. 1 and 2 illustrates the location above the touch surface that corresponds to the Predetermined Threshold Value such as to take into account the variability of other factors that could influence the touch sensitivity or “touch feel”. If the designer had to account for the use of gloves on a finger/appendage or other touch input device, then the location above the touch surface that would correspond to the Predetermined Threshold Value would have to be a greater distance to accommodate the thickness of the glove insulation. Of course when finger/appendage or other touch input device were to approach the touch surface, the Predetermined Threshold Processing Circuit 108 would register a valid touch event even though the finger/appendage or other touch input device would not actually be touching the touch surface. The corresponding location of the Predetermined Threshold Value could be right at the touch surface. In this case the designer would be taking into account the amount of signal contribution due to the flattening of the finger/appendage after initial contact to the touch surface. The stimulus signal continues to increase as the capacitive coupling of the finger to the glass increases which will causes the capacitance Ce in FIG. 3 to increase and the capacitance Ce in FIG. 8 to decrease. The designer has to take into account all variables that would affect what the Predetermined Threshold Value should be. It would be very important that after taking into account all of these variables that the Predetermined Threshold Value is not set to such a value such that when a finger/appendage or other touch input device is brought to the touch surface there would not be a valid touch event recognized. Conversely, the Predetermined Threshold Value should not be set as to cause false actuations. All of the variables above, including environmental conditions need to be taken into account to determine the proper compromise for setting the Predetermined Threshold Value.